Hydrocarbon conversion processes often employ multiple reaction zones through which hydrocarbons pass in a series flow. Each reaction zone in the series often has a unique set of design requirements. A minimum design requirement of each reaction zone in the series is the hydraulic capacity to pass the desired throughput of hydrocarbons that pass through the series. An additional design requirement of each reaction zone is sufficient heating to perform a specified degree of hydrocarbon conversion.
One well-known hydrocarbon conversion process can be catalytic reforming. Generally, catalytic reforming is a well-established hydrocarbon conversion process employed in the petroleum refining industry for improving the octane quality of hydrocarbon feedstocks, the primary product of reforming being a motor gasoline blending component or a source of aromatics for petrochemicals. Reforming may be defined as the total effect produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics, dehydrogenation of paraffins to yield olefins, dehydrocyclization of paraffins and olefins to yield aromatics, isomerization of n-paraffins, isomerization of alkylcycloparaffins to yield cyclohexanes, isomerization of substituted aromatics, and hydrocracking of paraffins. A reforming feedstock can be a hydrocracker, straight run, FCC, or coker naphtha, and can contain many other components such as a condensate or a thermal cracked naphtha.
Heaters or furnaces are often used in hydrocarbon conversion processes, such as reforming, to heat the process fluid before it is reacted. Generally, fired heaters or furnaces include an all radiant fired heating zone to heat the fluid with an optional convection section being used for another service, such as producing steam. Other fired heaters can have an initial convection section followed in a series by a radiant section. Having the convection section first allows for the process fluid to recover more heat from the flue gas because, generally, the convection section is at a lower temperature as compared to the radiant section of the heater. Additionally, both of these heater designs are applicable to charge heaters and interheaters. Each section includes tubes to contain the process fluid flowing through the heater.
However, these conventional designs suffer disadvantages. Sometimes a conversion unit is limited by the heater if increasing the firing of the heater raises the temperature of the radiant and/or convection tubes to their maximum tube wall limit. If the throughput of a heater is limited by a maximum tube wall temperature, then the production rate of the entire conversion unit can be constrained.
Moreover, generally there are three problems associated with operating a heater at or near the maximum temperature of the tube walls. First, high tube wall temperatures increase the tendency of flue gas to oxidize on the sides of the tubes, leading to the formation of scale that decreases the radiant efficiency of the heater. Second, high tube wall temperatures, particularly with respect to the first two reactors in a conversion process such as reforming, can cause cracking of the feed reducing yield. Third, an additional complication is that reforming heaters are also susceptible to having metal-catalyzed coking in the fired heater tubes at higher temperatures. Metal catalyzed coking can cause the shutdown of reforming units for maintenance work to remove the coke deposits in the reactors resulting from the onset of metal catalyzed coke formation in the fired heater tubes. As a result, lower tube wall temperatures are very desirable.
There are several solutions to coking problems associated with high tube wall temperatures, but each has its drawbacks:
a) sulfur can be injected that inhibits coke formation, but this solution generally decreases reformer yields and may be unnecessary for some feeds that do not tend to coke;
b) the radiant tubes can be replaced with tubes of different alloys that can raise the maximum allowable heater tube wall temperature, but these alloys tend to be more expensive;
c) the heater can be enlarged with more tubes and/or burners to increase surface area, but enlarging a heater is usually expensive; and
d) a heater can be added to the series of heaters to provide some of the required duty, so the size of the existing heater can be decreased. However, adding a heater is also usually expensive.
It has been considered very important to design fired heaters so that the distribution of fluid from the manifold across the set of parallel heater tubes is as uniform as possible. Problems arise with maldistribution of the fluid across the heater tubes. For example, the process outlet temperature of the heater overall is limited by the tube that rises to the highest tube wall temperature. If the first tube were to have a higher flow of fluid than that of the last tube, the last tube would reach an upper limit of tube wall temperature before the first tube would reach the upper limit.
Additionally, sometimes conversion units are refurbished during shutdowns to increase the capacity of the units. High fired heater tube wall temperatures can limit the potential feed rate increase or reformate octane increase for conversion units, such as reforming units. Such tube wall temperature limitations can result in the installation of large expensive fired heater cells. Such fired heater cells can be about 20% to 25% of the estimated cost of a conversion unit, such as a reforming unit.
When designing a fired heater for use in a new process, the size of the manifolds, the diameter of the heater tubes and other design variables are selected to best suit the process at hand. However, during refurbishment, many design variables are set, or changing the variables would lead to significant expense. For example, manifold size and tube diameter are costly to change at the time of refurbishment. Furthermore, analysis techniques of today may uncover problems that went previously undetected. For example, if the ratio of the pressure drop across the heater tubes to the pressure drop across the manifold was above a specific value, it was common engineering practice to assume under those conditions that uniform distribution was achieved. However, today's analyses show that is not necessarily true, especially in the case of refurbishments.
It has been discovered that the long held engineering assumptions were not always adequate and adjustments may need to be made to achieve uniform flow distribution across heater tubes. Some adjustments, such as increasing the size of the manifold, may be quite costly. However, once the problem was discovered to be maldistribution of flow across the heater tubes, applicants found an inexpensive corrective design change that involved placing a restriction orifice adjacent the inlet of at least one select heater tube. It is likely that a restriction orifice may be placed adjacent the inlet of multiple selected tubes or even all tubes. A restriction orifice may be placed adjacent the outlet of one more selected heater tubes to achieve the same result.
During refurbishment, a restriction orifice may be placed in between the inlet manifold and the inlet to the heater tube. Other embodiments include installing a restriction orifice within the inlet to the heater tube, or within the opening of the inlet or outlet manifold, or within in the inlet or outlet manifolds themselves, or any combination thereof.
In another embodiment of the invention, restriction orifices may be placed at the inlet of selected heater tubes to take advantage of hot-spots within a fired heater. In this case, a non-uniform flow distribution is desired and intentional. For example, heater tubes located towards the middle of the fired heater may receive heat from two sets of burners and be capable of heating fluid faster than other heating tubes. Therefore, the flow rate of fluid through these select tubes may be increased relative to the rest of the heater tubes with the resulting fluid still reaching the desired temperature. In this embodiment, those tubes not located in a hot spot may have a restriction orifice placed at the inlet of the heater tube in order to cause a greater flow rate through those heater tubes located within the hot spot of the heater.
Therefore, there is a desire to increase the feed through a conversion unit and not exceed the maximum tube wall temperature without incurring at least some of the disadvantages and costs discussed above. Correcting maldistribution through the use of at least one restriction orifice can help increase fluid through the fired heater without exceeding tube wall temperature limitations.